Ohanapecosh Formation
geologic formation in Washington, United States
The volcaniclastic Ohanapecosh Formation is an early state of Cascade volcanism. It has been dated to the middle Oligocene [36 to 28 Ma]. The strata are as much as 3 kilometres (1.9 miles) thick, with exposures visible in more than 400 square kilometres (99,000 acres) of a total area exceeding 700 square kilometres (170,000 acres). It is found throughout Mount Rainier National Park and the surrounding mountains. It is the foundation on which the Mount Rainier volcano was built. The formation has been identified in the Snoqualmie area to the north, and as far south as the Columbia River Gorge, including Mount St. Helens and Mount Adams. It extends from the west at Mount Rainier and Lake Tapps, east to Little Naches River valley.
Description
The contact of the Ohanapecosh Formation on the Puget Group is everywhere conformable and the Spiketon Formation and Renton Formation reflect a continuous process without a break in time. In contrast, the contact with the Naches Formation is an unconformity as seen in the Summit Creek Sandstone (~43 to 37 Ma) in the areas from White Pass east to the Naches River.
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This entry incorporates text from “Ohanapecosh Formation” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.